Developing softphone with call hold function

I needed to develop a softphone with call holding feature and I used this sample program http://www.voip-sip-sdk.com/p_353-hold-call-voip.html for this purpose provided by Ozeki VoIP SIP SDK. I could build this call holding function easily as they provided a very useful source code explanation as well.If you are interested in VoIP communication technology, on the website you can find many useful sample programs (which you can try for free for 20 days) and source codes too.